In modern cities, most areas are brightly illuminated even at night by many light sources including automobile headlights and store illuminations. On the other hand, most areas in suburbs and provincial districts are dark at night. Thus such areas as pathways, roadways, parks, and tunnels are provided with streetlights and outdoor luminaires for the safety and security purposes.
However, when high-intensity luminaires are installed in suburbs and provincial districts and turned on at night to produce bright illumination, there arise problems including nuisance to the neighborhood, excessive power consumption, and high installation costs. It would be desirable to have outdoor luminaires which can attain the purposes of safety, security and crime prevention without excessive power consumption.
When the light source for streetlights and outdoor luminaires is considered from the standpoints of maintenance and economy, LED lamps are of greater interest, because the LED lamps are advantageous in power consumption and lifetime over traditional incandescent lamps and fluorescent lamps. LED lamps come with a wide variation of light emission. Among them, blue LED lamps that produce light including a blue component may be used as the light source for outdoor luminaires, typically blue streetlights which are nowadays on widespread use. The power utility efficiency of LED lamps is very high. The power necessary for LED lamps to provide an equivalent illuminance is as low as ⅕ or less of the power consumed by incandescent lamps. LED lamps are generally believed to have a lifetime of several tens of thousands hours, leading to savings of labor and cost for maintenance service. For these reasons, LED lamps are suited for use in streetlights and outdoor luminaires which are intended for long-term continuous operation.
White LED packages for general lighting application have the structure that a phosphor is coated on a front surface of a blue LED chip having emission wavelength of 440 to 470 nm, or a phosphor layer is disposed immediately forward of a blue LED chip in emission direction. With this structure, blue light is irradiated from the chip to the phosphor, which in turn, emits broad luminescent light having a peak wavelength around 550 nm, which is combined with blue light of unconverted wavelength to produce white light. White LED lamps for most luminaires utilize luminescent light having a peak wavelength of about 550 nm because human eyes have a high sensitivity to light of wavelength near 555 nm. In principle, light rich in this wavelength component provides highly effective lighting. However, it is known that under scotopic vision conditions at night streets or under mesopic vision conditions immediately after sunset, the peak wavelength of human visual sensitivity shifts from 555 nm to the shorter wavelength side as shown in FIG. 7. This is known as the Purkinje effect.
Under scotopic and mesopic vision conditions, among outdoor luminaires using white LEDs, those having an emission peak shifted to a wavelength shorter than 555 nm are preferred. For night lighting, luminaires having a high color temperature are under study (Non-Patent Document 1). These prior art night-lighting luminaires are not regarded as complying with a change of visual sensitivity based on the Purkinje effect because the light does not fully contain wavelength components of the highest visual sensitivity at the scotopic and mesopic vision levels.